Oakland Athletics starting pitcher Jharel Cotton (45) throws a pitch in the third inning inning as the Houston Astros face the Oakland Athletics at Oakland Coliseum in Oakland, Calif., on Monday, September 19, 2016.

Cotton (3.75 ERA, 1.17 WHIP) had been in competition for one of three rotation vacancies, and after struggling through his first four appearances, allowing five hits and four earned runs in 4 innings, he had locked into a groove. In his two last starts of the spring, the right-handed changeup specialist held the opposition to three hits and one run combined, striking out five in 4 innings each time out.

The injury virtually assures Paul Blackburn (3.86 ERA, 1.07 WHIP) an Opening Day spot. It also opens the door for AJ Puk (3.38 ERA, 1.22 WHIP), the franchise’s No. 1 prospect (No. 32 league-wide) according to MLB.com, to slide into one of the three open slots.

Also battling for one of the spots are Daniel Mengden (8.04 ERA, 1.79 WHIP) and Andrew Triggs (4.63 ERA, 0.77 WHIP). One of the four will be sent to Triple-A Nashville when Cactus League play completes.

Kalama Hines is SFBay’s sports director and Oakland Athletics beat writer. Follow @SFBay and @HineSight_2020 on Twitter and at SFBay.ca for full coverage of A’s baseball.

Oakland Athletics starting pitcher Jharel Cotton (45) throws a pitch in the third inning inning as the Houston Astros face the Oakland Athletics at Oakland Coliseum in Oakland, Calif., on Monday, September 19, 2016.